Production method for sintered metal-ceramic layered compact and production method for thermal stress relief pad

ABSTRACT

The present invention provides a production method for a sintered metal-ceramic layered compact, comprising steps of: filling and layering a metal powder and a ceramic powder, or filling and layering a metal powder, a mixed powder of a metal powder and a ceramic powder, and a ceramic powder; forming a green compact of the layered powders by compacting the layered powders; and sintering a layer including the metal of the green compact at a temperature of lower than a melting point of the metal by heating by irradiation of microwaves in a non-oxidizing atmosphere.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a production method for producingmetal-ceramic layered compacts by using a powder metallurgy technique,and relates to a production method for producing thermal stress reliefpads for thermoelectric conversion elements by using a powder metallurgytechnique. The metal-ceramic layered compact is heat resistant andthermally conductive, or is electrically insulated at one portionthereof and electrically conductive and thermally conductive at anotherportion thereof, and has a required thermal stress relief function. Thethermal stress relief pad has a structure in which metal and ceramic arelayered and which is electrically conductive and electrically insulated.

2. Description of the Related Art

A metal-ceramic layered compact which is heat resistant and heatdissipating is disclosed in, for example, Japanese Unexamined PatentApplication Publication No. 5-286776, as described below. That is, ametal-ceramic layered compact has an intermediate layer between a metallayer and a ceramic layer. The metal layer is made of copper, nickel, ortungsten. The ceramic layer is made of alumina, aluminum nitride, orboron nitride. The intermediate layer is made of metal and ceramic suchthat the mixing ratio of metal and ceramic varies gradually orcontinuously in a thickness direction. This layered compact is producedas follows. That is, a metal included layer is layered by thermalspraying on a ceramic substrate or is layered by paste printing thereon,and is then sintered by hot pressing, by hot isostatic pressing (HIP),or by an electrization heating method in which voltage is directlyapplied thereto so that plasma discharge is generated among grainsthereof.

A metal-ceramic layered compact is disclosed in, for example, JapaneseUnexamined Patent Application Publication No. 6-329480, as describedbelow. The metal-ceramic layered compact has an intermediate layerbetween an alumina substrate and a copper plate. The intermediate layeris composed of tungsten, silver-copper alloy, and titanium. Thecomposition of the intermediate layer is set such that the silver-copperalloy is included more on the copper plate side. The intermediate layeris laminated by paste printing, and is then sintered in a vacuum or inan atmosphere of nitrogen gas, hydrogen gas, or argon gas.

In the above conventional techniques, the metal-ceramic layered compactis obtained such that metal layers are layered by thermal spraying or bypaste printing on a presintered ceramic substrate, and the metal layersare then sintered. The ceramic has high strength and the metal-ceramiclayered compact has good thermal conductivity since the metal layershave fine structures. However, in the above production methods, afterthe ceramic is sintered at high temperatures, the following processesare repeatedly performed. That is, a thermal spraying process issequentially performed on metal containing powders which have differentcompositions from each other, or pasted materials are printed, and adrying process is performed. Due to this, the above techniques requirenumerous processes, are time consuming, and are troublesome. As aresult, it is desired that a metal-ceramic layered compact be moreeasily produced.

A thermal stress relief pad for a thermoelectric conversion element isdisclosed in, for example, Japanese Unexamined Patent ApplicationPublication No. 10-229224 as described below. That is, the thermalstress relief pad has an electrical insulation layer made of ceramic atthe intermediate portion in a thickness direction, and metal layers areformed via a mixed layer of ceramic (electrical insulation material) andmetal (thermal stress relief material and thermal conductivity compact)on both sides of the electrical insulating layer. In this case, themixed layer is a graded function layer having a gradient component inwhich the ceramic is included more on the electrical insulating layerand the metal is included more on the metal layer. This thermal stressrelief pad is used such that one end face thereof is contacted to anelectrode side of the thermoelectric element and the other end face iscontacted to a side of a heat source or to a side of a cooling device.As a result, thermal stress relief pad has good thermal conductivity, isprevented from leaking electricity to the heat source side or to thecooling device by the electrical insulating layer, and yield a thermalstress relief function by having the graded composition in a thicknessdirection. For example, the mixture of the electrical insulatingmaterial and the metal is composed of alumina and copper, and isproduced such that powder filling is performed by injecting each powderfrom a nozzle into a die while controlling the injecting ratio thereofso as to have a graded composition in a thickness direction, and thencompacting and sintering are performed on the layered powders in thedie.

In the production method disclosed in the Japanese Unexamined PatentApplication Publication No. 10-229224, when a multilayered structurehaving a graded composition layer is produced, a method as describedbelow can be used instead of the multilayered filling method by theabove powder spraying. That is, one or more kinds of a mixed powder anda metal powder are layered, are filled by using a powder feeder, and areformed by compacting in turn in a die. The mixed powder is composed ofan electrical insulating powder (for example, alumina powder) and ametal powder (for example, copper powder). However, since a thermalstress relief pad for thermoelectric conversion elements generally has astructure such that a conductive metal-ceramic mixed layer and a metallayer are formed on both sides of the electrical insulating layer (forexample, alumina simple substance) which is at a middle portion in athickness direction thereof, when the powders are sequentially filledand layered, the metal powder enters an outer face of the electricalinsulating layer via an inner wall surface of the die. In this case,metal foil is formed on the outer face of the electrical insulatinglayer, and causes a short circuit in the thermal stress relief pad.

In addition, in the production method disclosed in Japanese UnexaminedPatent Application Publication No. 10-229224, a green compact has astructure such that the electrical insulating layer made of, forexample, alumina powder, and is disposed between the mixed powder ofalumina and copper. However, since this green compact is sintered at atemperature at which copper does not melt, an alumina compact of theelectrical insulating layer is not sintered well, and cracks therebyoccur at the electrical insulating layer portion. As a result, thesintered compact requires care in the use thereof.

SUMMARY OF THE INVENTION

An object of the present invention according to an aspect of theinvention is to provide a production method for a sintered metal-ceramiclayered compact, which can reduce the number of processes and can beperformed efficiently.

An object of the present invention according to another aspect of theinvention is to provide a production method for thermal stress reliefpads for thermoelectric conversion elements, in which, however,compacting is performed on the powders by using a die, can prevent ashort circuit which is caused by metal materials and can yield reliableperformance of an electrical insulating layer.

An object of the present invention according to another aspect of theinvention is to provide a production method for thermal stress reliefpads for thermoelectric conversion elements, which can reduce the numberof processes and can be performed efficiently.

The present invention provides a production method for a sinteredmetal-ceramic layered compact, comprising steps of: filling and layeringa metal powder and a ceramic powder, or filling and layering a metalpowder, a mixed powder of a metal powder and a ceramic powder, and aceramic powder; forming a green compact of the layered powders bycompacting the layered powders; and sintering a layer including themetal of the green compact at a temperature of lower than a meltingpoint of the metal by heating by irradiation of microwaves in anon-oxidizing atmosphere.

The present invention further provides a production method for asintered metal-ceramic layered compact, comprising steps of: filling andlayering a metal powder and a ceramic powder, or filling and layering ametal powder, a mixed powder of a metal powder and a ceramic powder, anda ceramic powder; forming a green compact of the layered powders bycompacting the layered powders; presintering a layer including the metalof the green compact at a temperature lower than a melting point of themetal by heating by irradiation of microwaves in a non-oxidizingatmosphere; and resintering the presintered compact at a temperaturelower than a melting point of the metal in a non-oxidizing atmosphere.

According to the present invention, the step of compacting the powdersand the step of sintering the green compact by using powder metallurgyare performed when the sintered metal-ceramic layered compact isproduced. As a result, the number of processes can be reduced and theproduction is performed efficiently.

The production method can use a microwave heating furnace provided witha cooling device, and a side of the metal layer of the compact may becontacted to the cooling device of the microwave heating furnace in thestep of sintering the green compact.

In the production method of the present invention, the metal may beselected from a group consisting of copper, aluminum, silver, andnickel, or a mixture thereof, and the ceramic may be alumina or aluminumnitride.

In the present invention, the following embodiments can be used. Theceramic powder may include at least one low melting point powderselected from a group consisting of boric acid, anhydrous borax, sodiumtriboric acid, sodium pentaboric acid, and soda-lime glass, and the lowmelting point powder may be mixed in a ratio of not more than 50 mass %in the ceramic powder. The ceramic powder may include at least onebinder selected from a group consisting of methyl cellulose (MC),polyvinyl alcohol (PVA), ammonium alginic acid, carboxymethyl cellulose(CMC), hydroxyethyl cellulose (HEC), and polyvinyl pyrrolidone (PVP),and the binder may be mixed in a ratio of not more than 1 mass % in theceramic powder. The mixed powder of the ceramic powder and the bindermay be granulated to have a particle diameter of not more than 150 μm.The mixed powder of the metal powder and the ceramic powder may have twoor more mixed powders which have different compositions from each other,wherein the metal may be mixed in a volume not less than that of theceramic powder in the mixed powder disposed on the side of the metallayer, and the ceramic powder may be mixed in a volume not less thanthat of the metal powder in the mixed powder disposed on the side of theceramic layer.

The present invention further provides a production method for asintered metal-ceramic layered compact, comprising steps of: filling andlayering a metal powder and a ceramic powder, or filling and layering ametal powder, a mixed powder of a metal powder and a ceramic powder, anda ceramic powder; forming a green compact of the layered powders bycompacting the layered powders; and sintering the green compact at atemperature lower than a melting point of the metal in a non-oxidizingatmosphere.

According to the present invention, the step of compacting the powdersand the step of sintering the green compact by using powder metallurgyare performed when the sintered metal-ceramic layered compact isproduced. As a result, the number of processes can be reduced and theproduction is performed efficiently.

The present invention further provides a production method for a thermalstress relief pad for thermoelectric conversion elements, comprisingsteps of: filling and layering an electrical insulating powder (30C) anda mixed powder (30B) of a metal powder and an electrical insulatingpowder in turn in a cavity of a die, and forming a green compact (31) ofthe layered powders by compacting the layered powders, or filling andlayering an electrical insulating powder (30C), a mixed powder (30B) ofa metal powder and an electrical insulating powder, and a metal powder(30A) in turn in a cavity of a die, and forming a green compact (32) ofthe layered powders by compacting the layered powders; and contacting anelectrical insulating layer, which is made of the electrical insulatingpowder (30C) in either the green compact (31) or the green compact (32),to a surface of an electrical insulating layer of the electricalinsulating powder (30C) in either the green compact (31) or the greencompact (32); or filling a mixed powder (30B) of a metal powder and anelectrical insulating powder in a cavity of a die, and forming a greencompact (33) by compacting the powder, and contacting an electricalinsulating layer, which is made of the electrical insulating powder(30C) in either the green compact (31) or the green compact (32), to asurface of the green compact (33), or filling and layering a metalpowder (30A) and a mixed powder (30B) of a metal powder and anelectrical insulating powder in turn in a cavity of a die, and forming agreen compact (34) of the layered powders by compacting the layeredpowders, and contacting an electrical insulating layer, which is made ofthe electrical insulating powder (30C) in either the green compact (31)or the green compact (32), to a surface of the green compact (34); andsintering the green compacts, which are in the above contacting state toeach other, at a temperature lower than a melting point of the includedmetal in a non-oxidizing atmosphere.

According to the present invention, the metal powder, the mixed powderof the metal powder and the electrical insulating powder, and theelectrical insulating powder are filled and layered in an appropriatemultilayered structure, two green compacts are thereby obtained, areappropriately combined with each other, and then are sintered at atemperature lower than the melting point of the included metal in anon-oxidizing atmosphere. As a result, short-circuiting caused by metalmaterials can be prevented and the electrical insulating layer canfunction reliably although compacting is performed on the powders byusing the die in the production method of the present invention.

The present invention further provides a production method for a thermalstress relief pad for thermoelectric conversion elements, comprisingsteps of: filling and layering a mixed powder (30B) of a metal powderand an electrical insulating powder, an electrical insulating powder(30C), and a mixed powder (30B) of a metal powder and an electricalinsulating powder in turn in a cavity of a die, or filling and layeringa metal powder (30A), a mixed powder (30B) of a metal powder and anelectrical insulating powder, an electrical insulating powder (30C), amixed powder (30B) of a metal powder and an electrical insulatingpowder, and a metal powder (30A) in turn in a cavity of a die; forming agreen compact of the layered powders by compacting the layered powders;sintering the green compact at a temperature lower than a melting pointof the included metal powder in a non-oxidizing atmosphere; and removinga side surface portion of the sintered compact by cutting or bypolishing.

In the present invention, the electrical insulating powder may be amixed powder (30C1), a mixed powder (30C2), or a glass frit powder(30C3), wherein the mixed powder (30C1) may be composed of one of analumina powder and an aluminum nitride powder, and one low melting pointelectrical insulating powder selected from a group consisting of boricacid, sodium boric acid, and soda-lime glass, the low melting pointelectrical insulating powder being mixed in a ratio of not more than 50mass %, the mixed powder (30C2) may be composed of one of an aluminapowder and an aluminum nitride powder and a glass frit which is mixed ina ratio of not less than 0.1 mass %, and the metal powder (30A) may beselected from a group consisting copper, aluminum, silver, and nickel,or a mixture thereof.

The present invention further provides a production method for a thermalstress relief pad for thermoelectric conversion elements, comprisingsteps of: filling and layering an electrical insulating material powder(40A) for an electrical insulating layer and a mixed powder (40B) of ametal powder and an electrical insulating material powder in a die, orfilling and layering an electrical insulating material powder (40A) foran electrical insulating layer, a mixed powder (40B) of a metal powderand an electrical insulating material powder, and a metal powder (40C)in a die; forming a green compact of the layered powders by compactingthe layered powders; and sintering the green compact at a temperaturelower than a melting point of the included metal powder in anon-oxidizing atmosphere, wherein the metal powder is selected from agroup consisting of copper, aluminum, silver and nickel, or a mixturethereof, the electrical insulating material powder (40A) is selectedfrom a group consisting of a glass frit (40A1) and a mixed powder (40A2)of a ceramic powder and a glass frit, the ceramic powder being composedof alumina or aluminum nitride, the electrical insulating materialpowder (40A) included in the mixed powder (40B) is selected from a groupconsisting of a ceramic powder, the glass frit (40A1), and a mixedpowder (40A2) of a ceramic powder and a glass frit.

According to the present invention, the step of compacting the powdersand the step of sintering the green compact by using powder metallurgyare performed when the sintered metal-ceramic layered compact isproduced. As a result, since the electrical insulating layer and themetal layer can be sintered simultaneously when the thermal stressrelief pad for thermoelectric conversion elements is produced, thenumber of processes can be reduced and the production is performedefficiently.

In the present invention, the electrical insulating material powder(40A) may be a mixed powder (40A2) of the ceramic powder and the glassfrit, and the glass frit may be mixed in a ratio of not less than 0.1mass % in the mixed powder (40A2). The following concrete methods can beused in the present invention. That is, the mixed powder (40B), theelectrical insulating material powder (40A) and the mixed powder (40B)may be layered in turn in the die in the step of filling and layeringpowders, or the metal powder (40C), the mixed powder (40B), theelectrical insulating material powder (40A), the mixed powder (40B), andthe metal powder (40C) may be layered in turn in the die in the step offilling and layering the powders, and the layered powders may beintegrally compacted in the step of compacting. Alternatively, the mixedpowder (40B) and the electrical insulating material powder (40A) may belayered in turn in the die in the step of filling and layering powders,or the metal powder (40C), the mixed powder (40B) and the electricalinsulating material powder (40A) may be layered in turn in the die inthe step of filling and layering powders, the layered powders areintegrally compacted in the step of compacting, whereby two greencompacts are obtained, and the green compacts may be sintered in a statein which surfaces of layers of the electrical insulating material powder(40A) are contacted to each other in the sintering step, thereby beingconnected.

The following concrete methods can be used in the present invention.That is, the electrical insulating material powder (40A) may include atleast one binder selected from a group consisting of methyl cellulose(MC), polyvinyl alcohol (PVA), ammonium alginic acid, carboxymethylcellulose (CMC), hydroxyethyl cellulose (HEC), and polyvinyl pyrrolidone(PVP), wherein the binder may be mixed in a ratio of not more than 1mass %. The binder may be mixed into the electrical insulating materialpowder (40A) in a middle portion layer in a thickness direction, and themixed powder may be granulated so as to have a particle diameter of notmore than 150 μm.

The present invention further provides a production method for a thermalstress relief pad for thermoelectric conversion elements, comprisingsteps of: filling a mixed powder (40B) of a metal powder and anelectrical insulating material powder in a die, or filling and layeringa mixed powder (40B) of a metal powder and an electrical insulatingmaterial powder, and a metal powder (40C) in turn in a die; forming agreen compact of the layered powders by compacting the layered powders,whereby two green compacts of the layered powders are obtained; coatingan electrical insulating material powder (40A) on a surface of a layerof the mixed powder (40B) of one of the green compacts; and connectingthe green compacts via the electrical insulating material powder (40A)by sintering. In this case, the electrical insulating material powder(40A) coated on a surface of a layer of the mixed powder (40B) may bedispersed in a liquid so as to be made into slurry.

In the both methods of the present invention, the mixed powder (40B) canhave two or more mixed powders which have different composition fromeach other, the metal powder (40C) can be mixed in a volume not lessthan that of the electrical insulating material powder (40A) on the sideof the metal layer formed on an end face, and the electrical insulatingmaterial powder (40A) can be mixed in a volume more than that of themetal powder (40C) in a electrical insulating layer formed at a middleportion in a thickness direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E are cross sectional views showing examples of amultilayered structure of a sintered ceramic-metal layered compactaccording to the first and the second embodiments.

FIGS. 2A and 2B are cross sectional views showing examples of a sinteredceramic-metal layered compact applied to a thermoelectric conversionmodule according to the first and the second embodiments.

FIG. 3 is a cross sectional view showing an example of a multilayeredstructure of a thermal stress relief pad according to the thirdembodiment.

FIG. 4 is a cross sectional view for explaining that a copper foilportion causing a short-circuit is formed at an electrical insulatinglayer of a sintered compact obtained by integrally compacting allpowders.

FIGS. 5A to 5D are cross sectional views showing examples of acompressed material.

FIGS. 6A to 6G are cross sectional views showing examples of a thermalstress relief pad.

FIG. 7 is a cross sectional view showing an example of a thermal stressrelief pad applied to a thermoelectric conversion module according tothe third embodiment.

FIGS. 8A to 8C are cross sectional views showing examples of amultilayered structure for thermal stress relief pads according to thefourth embodiment.

FIGS. 9A to 9C are cross sectional views showing examples of a thermalstress relief pad.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be describedhereinafter with reference to the Figures.

(A) First Embodiment

Desirable materials and production method in which the desirablematerials are used according to the first embodiment will be describedin detail hereinafter.

(1) Ceramic Powder

A Ceramic powder is composed of alumina or aluminum nitride, which hasgood electrical insulation and good thermal conductivity. In this case,in particular, alumina has better powder compression compactibility thanthat of aluminum nitride, and has a lower melting point than that ofaluminum nitride, thereby being favorably used. A ceramic powder as acomponent of a ceramic layer is favorably densified as high as possibleby compacting, and has good sinterability, thereby favorably having finegrain size. When a ceramic powder has low flowability due to fine grainsize thereof, the ceramic powder is favorably granulated by using abinder such as carboxymethyl cellulose (CMC) so as to have a particlediameter of about 50 to 150 μm, and the flowability thereof is therebyimproved. As a result, powder filling into a die is easily performed andthe ceramic powder compact has high strength. As compared with a fineceramic powder, a coarse powder is mixed with the fine ceramic powder,and the sinterability and flowability can thereby be improved. A ceramicpowder which is mixed with a mixture of metal and ceramic and which iscomponent of an intermediate layer favorably has the grain sizeapproximate to that of the metal powder so that the ceramic powder isequally dispersed in the metal powder and the metal powder is sintered.

(2) Low Melting Powder Added to Ceramic Powder

A ceramic layer of only alumina can be sintered by irradiation ofmicrowaves. A electrical insulating material, which is softened or has aliquid phase at temperatures at which layered metal does not melt, isformed into a powder and is mixed into a ceramic layer, whereby liquidphase sintering is performed on the ceramic layer at relatively lowtemperatures, thereby having high strength.

This low meting point powder is as follows.

-   a) boric acid (H₃BO₃): melting point of 577° C. in a state of    anhydrous boric acid-   b) anhydrous borax (Na₂B₄O₇): melting point of 741° C.-   c) sodium pentaboric acid (NaB₅O₈.5H₂O): melting point of 750° C.-   d) sodium triboric acid (NaB₃O₅): melting point of 694° C.-   e) soda-lime glass (SiO₂—Na₂O—CaO—Al₂O₃—MgO): softening point of 500    to 700° C.; melting point of about 725° C.

In the low melting point powder, plural low melting point materials andhigh melting point materials can be added to a ceramic layer. When theadding ratio of the low melting point powder is about 0.1 mass % in theceramic layer, the strength of the ceramic layer is improved. When theadding ratio of the low melting point powder is larger, the liquid phaseof the low melting point powder may possibly bubble to the surface ofthe ceramic layer in the case of sintering the ceramic layer or in thecase in which the temperature range of a sintered metal-ceramic layeredcompact is high, whereby the adding ratio of the low melting pointpowder is not more than 50 mass % in the ceramic layer.

(3) Binder for Ceramic Powder

A green compact of a ceramic layer has predetermined strength so as tobe easily handled by adjusting the grain size distribution of a ceramicpowder. A binder such as methyl cellulose (MC), polyvinyl alcohol (PVA),ammonium alginic acid, carboxymethyl cellulose (CMC), or polyvinylpyrrolidone (PVP) is mixed into a ceramic layer, or is mixed into aceramic powder for granulating, whereby the green compact can havehigher strength. As a result, when the green compact is transferred inprocesses of powder compacting and of sintering, cracks and defects canbe prevented from forming therein. Although the green compact can beproduced without the above binder, it is desirable that flowability ofthe ceramic powder be improved by granulating the ceramic powder so thatfilling to a die is improved.

The above binder dissipates when heated during the sintering of theceramic layer. Since the density of the ceramic layer is reduced and thethermal conductivity thereof is deteriorated when too much of the abovebinder is added much, the mixing ratio of the above binder into theceramic layer is favorably not more than 1 mass %.

(4) Powder of Metal Layer

A metal layer which is electrically conductive and thermally conductiveis made of a metal powder. The metal powder is composed of one ofcopper, aluminum, silver, and nickel, or mixture of at least two ofcopper, aluminum, silver, and nickel. For example, the mixture may becomposed of copper and aluminum. Although these powders have goodcompressibility, these powders favorably have predetermined grain sizesso as to pass through a 100-mesh sieve, thereby facilitating fillinginto a die. When a fine powder is used, flowability can be improved bygranulating.

(5) Powder of Intermediate Layer

An intermediate layer is made of a metal powder and a ceramic powder.The volume ratio of the metal powder to the ceramic powder is about 1:1.Alternatively, when plural intermediate layers of the metal powder andthe ceramic powder are formed, the component ratio of the metal powderto the ceramic powder increases as the intermediate layers approach themetal layer, and the component ratio of the ceramic powder to the metalpowder increases as the intermediate layers approach the ceramic layer.The mixed powders are made such that the ceramic powder is mixed intothe metal powder without granulation so that these powders are equallydispersed.

(6) Lubricant

It is not necessary to mix a lubricant into the metal powder because themetal powder has good compressibility. A lubricant such as a metalstearate is favorably coated on an inner wall of a die so that the greencompact is easily ejected from the die. The lubricant is coated byelectrostatic coating. Alternatively, the lubricant dispersed in aliquid is used.

(7) Multilayered Structure

A multilayered structure has a metal layer and a ceramic layer, has ametal layer, an intermediate layer and a ceramic layer, has a metallayer, an intermediate layer, a ceramic layer and an intermediate layer,or has a metal layer, an intermediate layer, a ceramic layer, anintermediate layer and a metal layer in turn on an end face of a layereddirection. The intermediate layer has at least one layer. The ceramiclayer has relatively low thermal conductivity, thereby being favorablythinly formed. However, when the ceramic layer is formed very thinly,the layers which are adjacent thereto and which include metal are easilymixed, and the electrical insulation may be possibly reduced. Therefore,the thickness of the ceramic layer is favorably about 0.5 to 2 mm.

(8) Multilayered Filling of Powders

A powder feeder can be used for filling each powder in a die having adie for forming an outer portion of a green compact, an upper punch anda lower punch. The powder feeder can be moved forward or backward on adie cavity. Plural powder boxes are connected to the powder feeder in apowder feeder moving direction. For example, when a multilayeredstructure has a metal layer, an intermediate layer, a ceramic layer, anintermediate layer and a metal layer, the powder feeder has three boxes.In this case, a metal powder is filled in the front box, an intermediatelayer powder is filled in the middle box, and a ceramic powder is filledin the rear box. The powder feeder is moved forward in a state in whichthe lower punch is flush with the upper face of the die, so that thepowder box having the metal powder is stopped on the lower punch, andthen the lower punch or the die is moved so as to form a cavity, wherebythe metal powder is filled therein. Next, the box having theintermediate layer powder is moved on the die cavity, and then theintermediate layer powder is filled in the same manner as that of themetal powder. After the ceramic powder is filled in the same manner asthat of the metal powder, the powder feeder is moved in turn backward,and multilayered filling of five layers can be performed. A powderfeeder has a structure such that spaces are provided between pluralpowder boxes. In this case, after one kind of powder is filled in thecavity, in a state in which the space is stopped on the cavity, thefilled powder is dropped, the cavity is formed, and the filled powderadhered on the wall surface of the die cavity is scratched and droppedby using a simple punch. As a result, a green compact having amultilayered structure which is distinctively divided can be obtained.

Since surfaces of the filled powders have microscopic rough portions,the powders adjacent to each other have slightly mixed portions of eachother. The intermediate layer and the ceramic layer have slightly mixedportions in the same manner. As a result, the compositions of the layersare not distinctively divided from each other, and the layers adjacentto each other are mixed so as to be connected to each other, and eachlayer is difficult to peel off from the green compact.

(9) Compacting of Powders

Compacting is performed on the metal powders of the above metal powdersas described below. That is, compacting is performed on the copperpowder, the silver powder, and the aluminum powder at a compactingpressure of about 100 to 300 MPa, and is performed on the nickel powderat a compacting pressure of about 400 MPa of the above metal powders,whereby the green compacts of these metal powders have the relativedensity of not less than 95% and thereby have good electricalconductivity and good thermal conductivity. On the other hand, whencompacting is performed on the ceramic powder of alumina at a compactingpressure of about 600 MPa, the green compact of the ceramic powder ofalumina has a relative density of about 50%. When compacting isperformed on the ceramic powder of alumina at a compacting pressure of700 MPa, the green compact of the ceramic powder of alumina has arelative density of about 60%. The compacting pressure of themultilayered powder is favorably about 700 to 1000 MPa since therelative density of the green compact of the ceramic powder graduallyincreases when compacting is performed on the ceramic powder of aluminaat a compacting pressure of more than 700 MPa.

(10) Microwave Sintering

A microwave sintering furnace is used for sintering. For example, asdisclosed in Japanese Unexamined Patent Application Publication No.6-345541, a microwave sintering furnace provided with a heater at aninner wall portion of a heating chamber can control preheating andcooling, thereby being favorably used. The inside of the heating chambercontains a non-oxidizing gas or is a vacuum when the green compacts aresintered. The non-oxidizing gas may be hydrogen, nitrogen, or argon, ormay be a mixed gas of hydrogen and nitrogen. When the metal powder iscomposed of silver, sintering can be performed in air. The furnace isconstructed such that a supporting pedestal and a holding plate on whicha sintered compact held is are provided in the heating chamber, and heatdischarging and cooling are performed thereon by a water cooling deviceprovided apart therefrom so that the green compact can be sintered at ahigh temperature at which the ceramic is sintered well without meltingmetal of the green compact, thereby being favorable when a metal powderof aluminum having a low melting point is used.

When microwaves are irradiated on a multilayered green compact of metaland ceramic, the ceramic is heated and the temperature thereof isincreased so that the degree of sintering of the ceramic is progressed.Since the metal portion of the multilayered green compact reflectsmicrowaves, the metal portion is not significantly heated by themicrowaves. However, the temperature of the metal portion is increasedby Joule heat, by heat conducted from the ceramic, and/or by radiantheat, so that the degree of sintering of the metal portion isprogressed. Since the shape thereof is collapsed when the intermediatelayer including the metal and the metal layer is melted, the output ofthe microwaves and processing time thereof are appropriately determinedby experience in accordance with the kind of metal and quantity of thegreen compact.

The lubricant and the binder are dissipated by microwave sintering, andthe ceramic layer, the intermediate layer, and the metal layer aresintered. When a low melting point powder such as a soda-lime glass isincluded in the ceramic layer, the low melting point powder is meltedwithout heating the ceramic layer to a high temperature, and the ceramiclayer is sintered and the interface portion between the ceramic layerand the intermediate layer has high bonding strength. In particular, amethod in which a low melting point powder is added to the ceramicpowder and aluminum having low melting point is used is favorable sincethe ceramic layer is sintered at a low temperature without melting thealuminum.

When microwave sintering is performed, the metal layer is electricallyconductive and thermally conductive, and wettability is ensured whenbrazing or adhering by an adhesive agent is performed in using thesintered multilayered compact.

(11) Resintering

Although in the above microwave sintering, a sintered multilayeredcompact of metal and ceramic can be obtained, when boric acid oranhydrous borax is included in the ceramic layer, a two step sinteringmethod can be adopted as described below. That is, microwave sinteringis briefly performed on the ceramic portion, the microwave sintering isstopped so that the metal portion is incompletely sintered, and then thepresintered compact is heated at a temperature at which the metal is notmelted in a non-oxidizing atmosphere. This sintering can be performed ina typical continuous sintering furnace, and is thereby suitable for massproduction. In the apparatus provided with the heater in the inner wallof the heating chamber as disclosed in the above Japanese UnexaminedPatent Application Publication No. 6-345541, resintering can beperformed on the presintered compact by heating by the heater afterstopping microwave irradiation.

Next, the first embodiment of the present invention will be describedwith reference to the Figures.

FIGS. 1A to 1E are cross sectional diagrams showing sinteredmetal-ceramic layered compacts. In the sintered metal-ceramic layeredcompacts, the metal layer is made of copper and the ceramic layer ismade of alumina.

A sintered layered compact 105A shown in FIG. 1A has a two-layeredstructure having a copper layer 103 and a ceramic layer 101 layered onthe copper layer 103. The copper layer 103 is made of an electrolyticcopper powder, and the ceramic layer 101 is made of a powder in whichanhydrous borax (Na₂B₄O₇) is mixed into alumina powder in a ratio of 1mass %. In producing the sintered layered compact 105A, the abovepowders are filled and layered in turn at a predetermined thickness in adie, and then compacting is performed on the filled layered powders at acompacting pressure of 800 MPa, whereby a green compact is obtained.Next, this green compact is provided in the microwave sintering furnace,nitrogen gas is charged therein, and microwaves are irradiated on thegreen compact so that the ceramic layer 101 is heated for five minutesat a temperature of about 900° C. and is then cooled. The primarysintered compact is resintered at a temperature of 800° C. under adissociated ammonia atmosphere in a mesh belt-type furnace, whereby thesintered layered compact 105A is obtained.

For example, this sintered layered compact is used as a heat dischargingmember. In this case, the ceramic layer 101 is contacted to a ceramicproduct or a ceramic member of which the temperature is increased, andheat dissipating fins are provided to the copper layer 103. When thecopper layer is needed having thermal conductivity and electricalconductivity and the ceramic layer 101 is needed having electricalinsulation, the copper layer 103 is contacted to a side which must beelectrically conductive and the ceramic layer 101 is contacted to a sidewhich must be electrically insulating. Since this sintered layeredcompact 105A has the copper layer 103 and the ceramic layer 101, theinterlayer therebetween is peeled off due to thermal expansiondifferences therebetween when this sintered layered compact 105A is usedin a high temperature atmosphere. Therefore, this sintered layeredcompact 105A is used at a relatively low temperature at which the abovephenomenon does not occur.

Sintered layered compacts 105B to 105E shown in FIGS. 1B to 1E will bedescribed hereinafter. Production methods for these sintered layeredcompacts 105B to 105E are the same as that for the sintered layeredcompact 105A. The following intermediate layer is made of a aluminapowder and a copper powder.

The sintered layered compact 105B is constructed such that twointermediate layers 121 and 123 having different composition from eachother are sandwiched between the copper layer 103 and the ceramic layer101. The intermediate layer 123 on the side of the copper layer 103 iscomposed of a mixed powder of an alumina powder and a copper powder. Themass ratio of the alumina powder to the copper powder is 15 to 85 in themixed powder of the intermediate layer 123. That is, the volume ratio ofthe alumina powder is about 30% in the mixed powder of the intermediatelayer 123. The intermediate layer 121 on the side of the ceramic layer101 is composed of a mixed powder of an alumina powder and a copperpowder. The mass ratio of the alumina powder to the copper powder is 30to 70. That is, the volume ratio of the alumina powder is about 50%.That is, the content of copper is large in the intermediate layer 123 onthe side of the copper layer 103, and the content of ceramic is large inthe intermediate layer 121 on the side of the ceramic layer 101. Thissintered layered compact 105B has a more advantageous structure than thesintered layered compact 105A in relieving thermal stress caused by heatcycling causing surrounding temperature changes and/or repeated thermalstresses.

The sintered layered compact 105C shown in FIG. 1C is constructed suchthat the ceramic layer 101 is sandwiched between two copper layers 103 aand 103 b. The copper layers 103 a and 103 b are thermally conductiveand electrically conductive, and the ceramic layer 101 therebetween iselectrically insulating. Therefore, the copper layers 103 a and 103 bare electrically insulated therebetween by the ceramic layer 101. Inthis sintered layered compact 105C, for example, heating is performed onthe copper layer 103 a and heat dissipation is performed on the copperlayer 103 b so as to have a cooling action. Since the ceramic layer 101directly contacts the copper layers 103 a and 103 b, heat resistancecharacteristics are inadequate, and the sintered layered compact 105C isfavorably used at a relatively low temperature or in an environmenthaving low temperature differences.

The sintered and layered compact 105D as shown in FIG. 5D is constructedsuch that an intermediate layer 122 a is sandwiched between the ceramiclayer 101 and the copper layer 103 a and an intermediate layer 122 b issandwiched between the ceramic layer 101 and the copper layer 103 b inthe sintered layered compact 105C. The volume ratio of alumina is about50% in the intermediate layers 122 a and 122 b. The sintered and layeredcompact 105E as shown in FIG. 5E is structured such that an intermediatelayer 121 a is sandwiched between the intermediate layer 122 a and theceramic layer 101, an intermediate layer 121 b is sandwiched between theintermediate layer 122 b and the ceramic layer 101, an intermediatelayer 123 a is sandwiched between intermediate layer 122 a and thecopper layer 103 a, and an intermediate layer 123 b is sandwichedbetween intermediate layer 122 b and the copper layer 103 b in thesintered layered compact 105D. The volume ratio of alumina is about 70%in the intermediate layers 121 a and 121 b. The volume ratio of aluminais about 30% in the intermediate layers 123 a and 123 b. As the above,three layers as intermediate layers are disposed between the ceramiclayer 101 and the copper layer 103 a and between the ceramic layer 101and the copper layer 103 b. In these sintered layered compacts 105D and105E, intermediate layers are disposed between the ceramic layer 101 andthe copper layer 103 a and between the ceramic layer 101 and the copperlayer 103 b, and these sintered layered compacts 105D and 105E havethermal stress relief, thereby giving good heat shock resistancecharacteristics.

Next, use examples of the above sintered layered compacts 105A to 105Ewill be described with reference to FIGS. 2A and 2B. In FIGS. 2A and 2B,reference numeral 105 shows one of the sintered and layered compacts105A to 105E.

FIG. 2A shows a cross sectional diagram of a thermoelectric conversionmodule 106A. The thermoelectric conversion module 106A is constructedsuch that plural N-type elements and plural P-type elements(thermoelectric elements 108) are positioned so as to alternate witheach other, the thermoelectric elements 108 are connected to each otherin a line by the sintered layered compacts 105, and the both ends of thesintered layered compacts 105 are sandwiched by metal plates 107 havinggood thermal conductivity so as to fix the members to each other. Forexample, the metal plates 107 may be copper plates. The sintered layeredcompact 105 may be used as a connecting pad. In this thermoelectricconversion module 106A, electricity is generated from a terminal mountedon the end of the thermoelectric element 108 by heating one side thereofand cooling the other side thereof. This thermoelectric conversionmodule 106A is mounted and used in a state of being disposed between aheat discharging portion of a furnace and a cooling device such as awater jacket.

In the thermoelectric module 106A as shown in FIG. 2A, thethermoelectric elements 108 and the sintered layered compacts 105 areconnected to each other by using solder or graphite coating, so thatelectrical conductivity and thermal conductivity therebetween areensured. The sintered layered compacts 105 and the copper plates 107 areconnected to each other by using solder or graphite coating, waterglass, or high melting point glass, so that thermal conductivitytherebetween is ensured. A thermoelectric conversion module 106B shownin FIG. 2B has the same fundamental structure as that of thethermoelectric conversion module 106A, and bolt 110 and nut 111 forfastening two copper plates 107 hold such that the members thereof arelayered and contacted to each other in the thermoelectric conversionmodule 106B.

The above sintered layered compacts 105A to 105E can be used as asintered layered compact 105 used in these thermoelectric conversionmodules 106A and 106B. In particular, the sintered layered compacts 105Dand 105E are favorably used since the copper layers 103 a and 103 b havegood electrical conductivity and thermal conductivity, the ceramic layerelectrically insulates between the copper layers 103 a and 103 b,thermal stress, which is caused by thermal expansion differences betweenthe high temperature side and the low temperature side and by heatcycling, can be relieved by the intermediate layers 121 a, 122 a, and123 a, and generation performance and reliability of thesethermoelectric conversion modules 106A and 106B are improved.

(B) Second Embodiment

Desirable materials and production method in which the desirablematerials are used according to the second embodiment will be describedhereinafter. In the second embodiment, description of the same materialsand structures as that of the first embodiment are omitted.

(1) Ceramic Powder

The same ceramic powder as that of the first embodiment is used.

(2) Low Melting Powder Added to Ceramic Powder

A ceramic powder composed of only ceramic powder is hardly sintered whenheated to a temperature in which the metal layer is sintered. Due tothis, the ceramic layer may possibly collapse when a strong impact isimparted thereto although the ceramic layer may be handled. Therefore,the same low melting point powder as that of the first embodiment isused so as to improve the strength of the ceramic layer.

(3) Binder for Ceramic Powder

The same binder for the ceramic powder as that of the first embodimentis used.

(4) Powder of Metal Layer

The same powder of the metal layer as that of the first embodiment isused.

(5) Powder of Intermediate Layer

The same powder of the intermediate layer as that of the firstembodiment is used.

(6) Lubricant

The same lubricant as that of the first embodiment is used.

(7) Multilayered Structure

The same multilayered structure as that of the first embodiment is used.

(8) Multilayered Filling of Powder

The same multilayered filling of the powders as those of the firstembodiment are performed.

(9) Compacting of Powders

The same compacting of the powders as those of the first embodiment areperformed.

(10) Sintering

Temperatures of metal-ceramic green compact are about 700 to 950° C.when the metal is copper, about 500 to 600° C. when the metal isaluminum, about 700 to 850° C. when the metal is silver, and about 800to 1150° C. when the metal is nickel. The inside of the heating chamberis in a non-oxidizing gas or in a vacuum when the green compact issintered. The non-oxidizing gas may be hydrogen, nitrogen, or argon, ora mixed gas of hydrogen and nitrogen. When the metal powder is composedof silver, the green compact can be sintered in air. The lubricant andthe binder are dissipated by sintering, and the ceramic layer, theintermediate layer, and the metal layer are sintered. When a low meltingpoint powder such as a soda-lime glass is included in the ceramic layer,the low melting point powder is melted without heating the ceramic layerto a high temperature, and the ceramic layer is sintered and theinterface portion between the ceramic layer and the intermediate layerhas high bonding strength. In particular, a method in which a lowmelting point powder is added to the ceramic powder and aluminum havinga low melting point is used is favorable since the ceramic layer issintered at a low temperature without melting the aluminum.

When sintering is performed, the metal layer is electrically conductiveand thermally conductive, and wettability when brazing or adhering by anadhesive agent is performed when using the sintered multilayered compactis ensured.

Next, the second embodiment of the present invention will be describedwith reference to the Figures.

In the second embodiment, a production method of the sintered layeredcompact 105A shown in FIG. 1A is different from that of the firstembodiment. That is, the green compact in the same manner as that of thefirst embodiment is provided in a sintering furnace, and is sintered ata temperature of 820° C. under a dissociated ammonia atmosphere in amesh belt-type furnace. As a result, the sintered layered compact 105Ais constructed in the above manner such that anhydrous borax is meltedand alumina is thereby sintered, thereby being integrally sintered andconnected. The above second embodiment can be applied to the sinteringmethods for the sintered layered compacts 105B to 105E shown in FIGS. 1Bto 1E.

(C) Third Embodiment

Desirable materials and production method in which the desirablematerials are used according to the third embodiment will be describedhereinafter.

(1) Metal Powder

A metal layer having electrical conductivity and thermal conductivity ismade of a metal powder. The metal powder is composed of one of copper,aluminum, silver, and nickel, or mixture of at least two of copper,aluminum, silver, and nickel. For example, the mixture is composed ofcopper and aluminum. These powders have good compressibility, and thesepowders favorably have predetermined grain sizes so as to pass through a100-mesh sieve, thereby being easily filled into a die. When a finepowder is used, flowability can be improved by granulating. These metalpowders used in a mixed powder with an electrical insulation powder areselected so that the mixed powder has low segregation degree and goodflowability in consideration of grain size distribution of theelectrical insulation powder, and commercial and common types used inproducing sintered alloy products can be used.

(2) Electrical Insulating Powder

A ceramic powder composed of alumina or aluminum nitride which has goodelectrical insulation characteristics and thermal conductivitycharacteristics can be used as a simple substance as a powder forforming an electrical insulating layer. Since a sintering of greencompacts is performed at temperatures lower than the melting point ofincluded metal therein, a material which is softened or is melted at asintering temperature and has electrical insulation is favorably mixedinto a ceramic powder in the form of a powder, the electrical insulatinglayer has high strength, and connecting layers thereto is reliablyperformed. For example, this low melting point powder is boric acid(melting point of 577° C. in a state of anhydrous boric acid), anhydrousborax (melting point of 741° C. in a state of anhydrous borax), orsoda-lime glass (softening point of 500 to 700° C.; melting point ofabout 725° C.). When the adding ratio of the low melting point powder isabout 0.1 mass % in the ceramic layer, the strength of the ceramic layeris improved. When the adding ratio of the low melting point powder islarger, the liquid phase of the low melting point powder may possiblybubble to the surface of the ceramic layer in a case of sintering theceramic layer, whereby the adding ratio of the low melting point powderis not more than 50 mass % in the ceramic layer. In the low meltingpoint powder, plural low melting point materials and high melting pointmaterials can be added to a ceramic layer. When the adding ratio of thelow melting point powder is about 0.1 mass % in the ceramic layer, thestrength of the ceramic layer is improved. When the adding ratio of thelow melting point powder is larger, the liquid phase of the low meltingpoint powder may possibly bubble to the surface of the ceramic layer ina case of sintering the ceramic layer, whereby the adding ratio of thelow melting point powder is favorably not more than 50 mass % in theceramic layer.

The other low melting point material is a glass frit. The glass frit asa glaze for enamel has a vitreous structure composed of SiO₂ as a maincomponent, B₂O₃, MgO, Al₂O₃, and BaO. In the present invention, othercommercial kinds of glass frit can be used. The glass frit is melted attemperatures of about 500 to 900° C., and is selected depending on themetal powder used. When the glass frit is added to a ceramic powder at aratio of 0.1 mass %, the ceramic powder is sintered by melting of theglass frit. As the ratio of the glass frit contained increases, thecontent of the liquid phase thereof increases in sintering theelectrical insulating layer at melting temperatures of the glass frit.In a case in which the content of the liquid phase of the glass frit isextensively generated much, the ceramic powder functions as a frame ofthe layer, whereby distortion of the layer is inhibited. The electricalinsulating layer can be made of only the glass frit in a case in whichthe sintering temperature is relatively low, the glass frit having arelatively high melting point is used, or the electrical insulatinglayer is formed thinly.

Since the glass frit and the mixed powder of the glass frit and theceramic powder are hard and have low compactibility, a binder such asmethyl cellulose (MC), polyvinyl alcohol (PVA), ammonium alginic acid,carboxymethyl cellulose (CMC), or polyvinyl pyrrolidone (PVP) is mixedinto an electrical insulating layer, whereby the green compact can havehigher strength. As a result, when the green compact is transferred inprocesses of powder compacting and of sintering, cracks and defects canbe prevented from occurring therein. The above binder dissipates whenheated in sintering the electrical insulating layer. Since the densityof the electrical insulating layer is reduced and the thermalconductivity thereof is deteriorated when too much of the above binderis added, the mixing ratio of the above binder in the electricalinsulating layer is favorably not more than 1 mass %.

Since the glass frit and the ceramic powder have relatively lowflowability, the flowability can be improved by granulating so thatpowder filling to a die is improved. When the above powders have lowflowability due to fine grain size thereof, the above powders arefavorably granulated by using a binder such as carboxymethyl cellulose(CMC) so as to have a particle diameter of about 50 to 150 μm, andflowability thereof is thereby improved. As a result, powder filling toa die is easily performed and a green compact has high strength. Ascompared with a fine ceramic powder, a coarse powder is mixed with thefine powder, sinterability and flowability can thereby be improved.

(3) Mixed Powder of Metal Powder and Electrical Insulating Powder

A mixed powder is formed into a functionally gradient layer. Forexample, the mixed volume ratio of the electrical insulating powder tothe metal powder is 1 to 1 in the functionally gradient layer.Alternatively, when the mixed layer is made to have plural layers, amixed powder including the electrical insulating powder is substantiallypositioned on the electrical insulating layer, and a mixed powderincluding the metal powder is substantially positioned away from theelectrical insulating layer. For example, when the mixed layer has threelayers, the mixed volume ratio of the electrical insulating powder tothe metal powder is 75 to 25 in the layer on the side of the electricalinsulating layer, is 50 to 50 in the intermediate layer, and is 25 to 75in the layer on the side of the metal layer.

(4) Lubricant

Since the electrical insulating powder is hard, a lubricant such as ametal stearate is favorably added to the electrical insulating powder bynot more than about 0.5 mass %, or is coated on an inner wall of a dieso that the green compact is easily ejected from the die. The lubricantis coated by electrostatic coating. Alternatively, a lubricant dispersedin liquid is used.

(5) Filling and Layering of Powders

A powder feeder can be used for filling each powder in a die having adie for forming an outer portion of a green compact, an upper punch anda lower punch. The powder feeder can be moved forward or backward on adie cavity. Plural powder boxes are connected to the powder feeder in apowder feeder moving direction. For example, when a multilayeredstructure has a metal layer, a mixed layer, an electrical insulatinglayer, a mixed layer, and a metal layer, the powder feeder has threeboxes. In this case, a metal powder is filled in the front box, a mixedlayer powder is filled in the middle box, and an electrical insulatingpowder is filled in the rear box. The powder feeder is moved forward ina state in which the lower punch is flush with the upper face of thedie, so that the powder box having the metal powder is stopped on thelower punch, and then the lower punch or the die is moved so as to forma cavity, whereby the metal powder is filled therein. Next, the boxhaving the mixed layer powder is moved on the die cavity, and then themixed powder is filled in the same manner as that of the metal powder.After the electrical insulating powder is filled in the same manner asthat of the metal powder, the powder feeder is moved in turn backward,and multilayered filling of five layers can be performed.

A powder feeder has a structure such that spaces are provided betweenplural powder boxes. In this case, after one kind of powder is filled inthe cavity, in a state in which the space is stopped on the cavity, thefilled powder is dropped, the cavity is formed, and the filled powderadhered on the wall surface of the die cavity is scratched and droppedby using a simple punch. As a result, a green compact having amultilayered structure which is distinctively divided can be obtained.

Since surfaces of the filled powders have microscopic rough portions,the powders adjacent to each other have slightly mixed portion with eachother. As a result, the compositions of the layers are not distinctivelydivided from each other, and the layers adjacent to each other are mixedso as to be connected to each other, and each layer is difficult to peeloff from the green compact.

(6) Compacting of Powders

Compacting is performed on the metal powders of the above metal powdersas described below. That is, compacting is performed on the copperpowder, the silver powder, and the aluminum powder at a compactingpressure of about 100 to 300 MPa, and is performed on the nickel powderat a compacting pressure of about 400 MPa of the above metal powders,whereby the green compacts of these metal powders have a relativedensity of not less than 95% and thereby have good electricalconductivity and good thermal conductivity. On the other hand, whencompacting is performed on the electrical insulating powder of aluminaat a compacting pressure of about 600 MPa, the green compact of theelectrical insulating powder of alumina has a relative density of about50%. When compacting is performed on the electrical insulating powder ata compacting pressure of 700 MPa, the green compact of the electricalinsulating powder has a relative density of about 60%. The compactingpressure of the multilayered powders is favorably about 700 to 1000 MPasince the relative density of the green compact of the electricalinsulating powder gradually increases when compacting is performed onthe electrical insulating powder at a compacting pressure of more than700 MPa.

(7) Multilayered Structure

A multilayered structure of a thermal stress relief pad has anelectrical insulating layer which is at a middle portion in a thicknessdirection and mixed layers which are formed on the both sides of theelectrical insulating layer. The electrical insulating layer has athickness of 0.5 to 2 mm so that thermal conductivity and electricalinsulation of the electrical insulating layer are ensured. The mixedlayer is layered such that the content of the electrical insulatingpowder is large on the side of the electrical insulating layer and thecontent of the metal powder is large on the side away from theelectrical insulating layer. Alternatively, a thermal stress relief padhas a multilayered structure such that the metal layer is provided on atleast one of the outsides of the mixed layer. One metal layer is used asan electrode which connects thermoelectric conversion elements. Whenelectrodes are separately produced and thermoelectric conversionelements are assembled, the multilayered structure may have no metallayer.

(8) Sintering

The same continuous sintering furnace as that used in producing sinteredmetal products is used in sintersing. Alternatively, microwave sinteringor plasma sintering can be performed. A typical mesh belt-typecontinuous furnace is favorable since sintering can be performedefficiently. Sintering is performed in a non-oxidizing gas or in avacuum when the green compacts are sintered. The non-oxidizing gas ishydrogen, or nitrogen, argon, or a mixed gas of hydrogen and nitrogen.When the metal powder is composed of silver, the green compact can besintered in air. The temperatures of sintering is about 700 to 950° C.when the metal is copper, is about 500 to 600° C. when the metal isaluminum, is about 700 to 950° C. when the metal is silver, and is about800 to 1150° C. when the metal is nickel. The kind of enamel frit forsintering or for melting is selected in accordance with the abovetemperature range.

The lubricant and the binder are dissipated by sintering, the metallayer, the mixed layer, and the electrical insulating layer aresintered, and each interlayer therebetween is strongly connected to eachother. The glass frit of the electrical insulating layer is sintered ormelted, is enameled and is adhered closely to the mixed layer. When theelectrical insulating powder of the mixed layer is only the ceramicpowder, the ceramic dispersed sintered metal composite material isformed. When only glass frit is included in the electrical insulatingpowder in the mixed layer, the glass frit is softened or melted, wherebysintering the mixed layer can be performed more quickly.

Next, the third embodiment of the present invention will be describedhereinafter with reference to Figures. FIG. 3 is a cross sectionaldiagram showing a thermal stress relief pad 301A for thermoelectricconversion elements. The thermal stress relief pad 301A has anelectrical insulating layer 302 and mixed layers 303 having plural mixedlayers layered on the both sides of the electrical insulating layer 302,metal layers 304 which are made of only copper and are layered as anoutermost layer thereof. The mixed layer 303 has a structure such that afirst mixed layer 331, a second mixed layer 332, and a third mixed layer333 are layered in turn on the side of the electrical insulating layer302. The content volume ratio of copper is small in the first mixedlayer 331, the content ratio of copper to electrical insulating materialis 1:1 in the second mixed layer 332, and the content of copper is largein the third mixed layer 333.

The following powders are used for producing this thermal stress reliefpad 301A.

-   -   (a) copper powder (for forming the metal layer 304)    -   (b) mixed powder (for forming the first mixed layer 331) of a        copper powder and an alumina powder (ratio of the copper powder        to the alumina powder is 50:50, that is, the volume ratio of        alumina is about 70%)    -   (c) mixed powder (for forming the second mixed layer 332) of a        copper powder and an alumina powder (ratio of the copper powder        to the alumina powder is 30:70, that is, volume ratio of alumina        is about 50%)    -   (d) mixed powder (for forming the second mixed layer 333) of a        copper powder and an alumina powder (ratio of the copper powder        to the alumina powder is 15:85, that is, volume ratio of alumina        is about 30%)    -   (e) ceramic powder (for forming the electrical insulating layer        302) composed of an electrical insulating powder of an alumina        powder and an enamel frit; the electrical insulating powder        including methyl cellulose at ratio of 0.1 mass % (weight ratio        of the alumina powder to the enamel frit powder is 1:1)

A vitreous powder composed of SiO₂ and/or B₂O₃ as a main component isused as an enamel frit. SiO₂ and/or B₂O₃ start melting at a temperatureof 700° C., and show a melted state in which they are wet and spread onthe copper plate when heated on the copper plate under a dissociatedammonia gas.

Next, the above powders are filled in turn into a cavity for a die in alayering direction, and then compacting is performed on the multilayeredpowders at a compacting pressure of 700 to 1000 MPa, whereby a greencompact is obtained. In this case, when the above powders are filledinto the die, a zinc stearate powder is coated by electrostatic coatingon an inner wall of the cavity, and then the above powders are filledthereto in turn by using a feeder. After compacting is simultaneouslyperformed on all the multilayered powders in the above manner, the greencompact is ejected from the die, and is then sintered. For example, thesintering is performed on the green compact by heating at a temperatureof 800° C. under a dissociated ammonia gas.

When the above processes of multilayered filling, compacting, andsintering are performed, a sintered compact shown in FIG. 4 is oftenobtained. That is, a thin copper foil portion 305 is formed in thissintered compact on a surface of a side of the electrical insulatinglayer 302 (on a side of the inner wall face of the die). The mixedlayers 303 are electrically short-circuited by the copper foil portion305, and the electrical insulating layer 302 does not perform anelectrical insulating function. The reason that the copper foil portion305 is formed is thought to be that, when multilayered filling isperformed on the above powders in the die in turn, the metal of themetal powder containing layer, which is filled before the electricalinsulating powder is filled therein, is adhered to the inner wall faceof the die, and then the electrical insulating layer is moved thereto,whereby the side of the electrical insulating layer 302 is covered withthe metal powder.

Therefore, the following stepwise production method is used formaintaining the electrical insulating function of the electricalinsulating layer 302.

FIGS. 5A and 5B show raw materials 310 a and 310 b of the green compact,which are referred to simply as “compressed materials 310 a and 310 b”.The electrical insulating powder, the various mixed powders, and thecopper powder are layered, are filled, and are compacted, whereby thecompressed material 310 a is obtained. The compressed material 310 a isa green compact having the electrical insulating layer 302 and the mixedlayer 303 which has the first mixed layer 331, the second mixed layer332 and the third mixed layer 333 in turn from the bottom, and the metallayer 304. Since compacting is performed in a state in which theelectrical insulating layer 302 is positioned at bottom, the side of theelectrical insulating layer 302 is not contaminated by the mixed powdersand/or the metal powders and is not adhered thereby when the abovecompacting is performed, whereby forming the copper foil portion 305shown in FIG. 4 is prevented and the side of the electrical insulatinglayer 302 is exposed. Next, the mixed layer 303 of the compressedmaterial 310 b is contacted to the side of the electrical insulatinglayer 302 of the compressed material 310 a, and then sintering isperformed thereon while maintaining the state of contact thereof. As aresult, contacting interfaces of the mixed layer 303 of the compressedmaterial 310 b and the electrical insulating layer 302 of the compressedmaterial 310 a are connected to each other, and the thermal stressrelief pad 301A shown in FIG. 6A is produced. According to this thermalstress relief pad 301A, the electrical insulating function of theelectrical insulating layer 302 is ensured and electricalshort-circuiting does not occur since forming the copper foil portion305 shown in FIG. 4 is prevented on the side of the electricalinsulating layer 302 of the compressed material 310 a.

The other method for securing the electrical insulating characteristicsof the electrical insulating layer 302 is as follows. That is, as shownin FIG. 4, a surface portion P-P′which is thicker than the copper foilportion 305 on a side of the sintered compact is removed by cutting orby polishing. As a result, the copper foil portion 305 causing ashort-circuit is removed, and the electrical insulating layer 302 isexposed on the side of the thermal stress relief pad 301A.

The thermal stress relief pad 301A shown in FIGS. 3 and 5A is oneexample of the present invention, and FIGS. 6B to 6G show thermal stressrelief pads 301B to 301G. FIGS. 5C and 5D show other compressedmaterials 310 c and 310 d.

The electrical insulating powder and the various mixed powders arefilled, are layered, and are compacted, whereby the compressed material310 c is obtained. The compressed material 310 c shown in FIG. 5C is agreen compact having the electrical insulating layer 302 and the mixedlayer 303 which has the first mixed layer 331, the second mixed layer332, and the third mixed layer 333 in turn from the bottom. Theelectrical insulating powder and the various mixed powders are filled,are layered, and are compacted, whereby the compressed material 310 d isobtained. The compressed material 310 d shown in FIG. 5D is a greencompact having only the mixed layer 303 which has the first mixed layer331, the second mixed layer 332 and the third mixed layer 333 in turnfrom the bottom.

Two kinds of compressed materials are appropriately selected from thecompressed materials 310 a to 310 d shown in FIGS. 5A to 5D, andsintering is performed on the selected kinds of compressed materials,whereby the thermal stress relief pads 301B to 301G shown in FIGS. 6B to6G can be produced. The thermal stress relief pad 301B shown in FIG. 6Bis produced such that two compressed materials 310 c are stacked so thatthe electrical insulating layers 302 thereof contact each other and aresintered. The thermal stress relief pad 301C shown in FIG. 6C isproduced such that compressed materials 310 a and 310 c are stacked sothat the electrical insulating layers 302 thereof contact each other andare sintered. The thermal stress relief pad 301D shown in FIG. 6D isproduced such that the electrical insulating layer 302 of the compressedmaterial 310 c and the compressed material 310 d are stacked on eachother and are sintered. The thermal stress relief pad 301E shown in FIG.6E is produced such that the electrical insulating layer 302 of thecompressed material 310 c and the mixed layer 303 of the compressedmaterial 310 b are stacked and are sintered. The thermal stress reliefpad 301F shown in FIG. 6F is produced such that the electricalinsulating layers 302 of the compressed materials 310 a are stacked andare sintered. The thermal stress relief pad 301F shown in FIG. 6F isproduced such that the electrical insulating layer 302 of the compressedmaterial 310 a and the compressed material 310 d are stacked and aresintered.

In the thermal stress relief pads 301B to 301G, two of the compressedmaterials 310 a to 310 d which are compacted beforehand areappropriately selected and sintered in the same manner as the case ofthe thermal stress relief pad 301A, whereby formation of the copper foilportion 305 which may cause a short-circuit is prevented, and theelectrical insulating function of the electrical insulating layer 302 issecured.

A use example of the above thermal stress relief pad 301A will bedescribed hereinafter with reference to FIG. 7. The thermal stressrelief pads 301B to 301G can be used appropriately instead of thethermal stress relief pad 301A.

FIG. 7 shows a cross sectional diagram of a thermoelectric conversionmodule 307. The thermoelectric conversion module 307 is constructed suchthat plural N-type elements and plural P-type elements (thermoelectricelements 305) are positioned so as to alternate with each other, thethermoelectric elements 305 are connected to each other in series by themetal layers 304 of the thermal stress relief pads 301A, and the bothends of the thermal stress relief pads 301A are sandwiched by metalplates 306 having good thermal conductivity so as to fix the members toeach other. For example, the metal plates 306 are copper plates.

The thermal stress relief pads 301A are connected to the thermoelectricconversion elements 305 by using solder or a graphite coating so thatelectrical conductivity and thermal conductivity therebetween areensured, and are connected to the copper plates 306 by using solder or agraphite coating, water glass, or high melting point glass, so thatthermal conductivity therebetween is ensured. Alternatively, instead ofusing the above adhesive agents, a bolt and a nut for fastening twocopper plates 306 hold such that the members thereof are layered andcontacted to each other in the thermoelectric conversion module 307. Inthis thermoelectric conversion module 307, electricity is generated froma terminal mounted on the end of the thermoelectric element 305 byheating one side thereof and cooling the other side thereof. Thisthermoelectric conversion module 307 is mounted and used in a state ofbeing disposed between a heat discharging portion of a furnace and acooling device such as a water jacket.

When the thermoelectric conversion module 307 is used, the metal layer304 contacting the thermoelectric conversion element 305 of the thermalstress relief pad 301A is an electrode member and a heat conductingmember. The electrical insulating layer 302 prevents electrical leakageto the sides of the copper plates 306. The thermal expansion coefficientof the mixed layer 303 is different from that of the metal layer 304 orthe copper plates 306. As a result, thermal stress, which is caused bythermal expansion difference between the high temperature side and thelow temperature side and by heat cycling, can be relieved and generationperformance and reliability of the thermoelectric conversion module 307is improved.

(D) Fourth Embodiment

Desirable materials and production method in which the desirablematerials are used according to the fourth embodiment will be describedhereinafter. In the fourth embodiment, description of the same materialsand structures as that of the third embodiment are omitted.

(1) Metal Powder

The same metal powder as that of the third embodiment is used. The samepowder, which is mixed into an electrical insulating material powder asthat of the third embodiment, is used. The electrical insulatingmaterial powder is composed of the following ceramics powder and thefollowing glass frit, and is used instead of the electrical insulatingpowder, which is composed of the ceramics and the low melting pointmaterial such as boric acid or is composed of the ceramics and the glassfrit, of the third embodiment.

(2) Ceramic Powder

A ceramic powder is composed of alumina or aluminum nitride, which hasgood electrical insulation and good thermal conductivity. In this case,in particular, alumina has better powder compression compactibility thanthat of aluminum nitride, and has a lower melting point than that ofaluminum nitride, thereby being favorably used. The ceramic powder isused as a mixed powder with a metal powder or as described below glassfrit. When the ceramic powder is added to the mixed powder, the ceramicpowder favorably has a grain size approximate to that of the metalpowder so that the ceramic powder is equally dispersed in the metalpowder and the metal powder is sintered.

(3) Glass Frit

The glass frit has a vitreous structure composed of SiO₂, B₂O₃, P₂O₅,Al₂O₃, or ZnO as a main component, and includes MgO, TiO₂, BiO₂, or CaOif necessary. The glass frit does not have electrical conductivity. Forexample, the glass frit may be an oxide glass which is widely used as aglass in practice, special glass such as an oxidized glass in which apart of oxygen is substituted by nitrogen, glaze used for enamel,cloisonné and ceramic, solder glass used for sealing or adhering, orbinder for a baking finish. Various kinds of the above glass frits aresold commercially. For example, a glass frit for a porcelain covering isdisclosed in Japanese Unexamined Patent Application Publication No.61-297, and glass frits for enamel substrates are disclosed in JapaneseUnexamined Patent Application Publication No. 3-63162, in JapaneseUnexamined Patent Application Publication No. 58-104042, in JapaneseUnexamined Patent Application Publication No. 3-73158, in JapaneseUnexamined Patent Application Publication No. 6-56923 and in JapaneseUnexamined Patent Application Publication No. 7-30463 in which componentof enamel is disclosed.

The glass frits have softening points of not less than about 350° C. Inconsideration of viscosity of the glass frit when softened and melted,wettability of the glass frit with metal and thickness of the electricalinsulating layer, the kind of the glass frit is selected from glassfrits having softening points of about 500 to 900° C., and whether ornot only glass frit is used and whether a ceramic is mixed into theglass frit are determined depending on sintering temperature of themetal for thermal stress relief pads. Borate glass or glaze for enamelis favorably used from a standpoint of adhesiveness thereof with metal.

(4) Powder for Forming Electrical Layer

A glass frit as a simple substance or a mixture of a ceramic powder anda glass frit is used as a powder for forming the electrical insulatinglayer. When the glass frit as a simple substance is used, the electricalinsulating layer is sintered at a temperature at which the glass frit ismelted and flowed freely, whereby the glass frit is flowed out of theouter portion of the multilayered compact so that the electricalinsulating layer is made much thinner. In this case, since there may bea case in which the electrical insulating layer breaks, the sinteringtemperature thereof is not more than the softening point thereof. Whenthe electrical insulating layer is sintered at a temperature in whichthe glass frit is melted, the glass frit is favorably mixed with aceramic powder such as an alumina powder or aluminum nitride. As aresult, the ceramic powder functions as a frame of the electricalinsulating layer so as to maintain the melted glass frit, the electricalinsulating layer is sintered, and the electrical insulating layer andthe layers adjacent thereto are connected reliably. When the glass fritis added to the ceramic powder at a ratio of 0.1 mass %, the greencompact of the ceramic powder is sintered in a state in which the glassfrit is in a liquid phase. When the included ratio of the glass frit islarger, the liquid phase of the glass frit increases by sintering, theelectrical insulating layer is sintered well and is strongly adhered tothe composite layers adjacent thereto.

(5) Binder for Forming Electrical Insulating Layer and GranulatingThereof

Since the glass frit and the mixed powder of the glass frit and theceramic powder are hard and are relatively fine, these materials havelow strength in the green compact, and care in handling is needed.Therefore, the same binder as that of the third embodiment is used, andthe same method of granulation as that of the third embodiment is usedso that the green compact has high strength.

(6) Mixed Powder of Metal Powder and Electrical Insulating Powder

A mixed powder is formed as an graded function layer. The mixed powderis a mixed powder of the metal powder and the ceramic powder, a mixedpowder of the metal powder and the glass frit, or a mixed powder of themetal powder, the ceramic powder and the glass frit. For example, themixed volume ratio of the metal powder to the electrical insulatingpowder is 1 to 1 in the mixed layer. Alternatively, when the mixed layeris made to have plural layers, a mixed powder including the electricalinsulating material powder is substantially positioned on the electricalinsulating layer, and a mixed powder including the metal powder issubstantially positioned away from the electrical insulating layer. Forexample, when the mixed layer has three layers, the mixed volume ratioof the electrical insulating material powder to the metal powder is 75to 25 in the layer on the side of the electrical insulating layer, is 50to 50 in the intermediate layer, and is 25 to 75 in the layer on theside of the metal layer.

(7) Lubricant

Since the electrical insulating material powder is hard, a lubricantsuch as a metal stearate is favorably coated on an inner wall of a dieso that the green compact is easily ejected from the die. The lubricantis coated by electrostatic coating. Alternatively, the lubricantdispersed in a liquid is used.

(8) Filling and Layering of Powders

The same filling and layering of powders as those of the thirdembodiment are used other than using the electrical insulating materialpowder instead of the electrical insulating powder of the thirdembodiment.

(9) Compacting of Powders

The same compacting of powders as those of the third embodiment areused.

(10) Multilayered Structure

The multilayered structure is shown in (a) to (f). The mixed powder ofthe metal powder and the electrical insulating material powder includesa powder having one kind of component or more kinds thereof.

-   (a) mixed layer-   (b) metal layer-mixed layer-   (c) mixed layer-electrical insulating layer-   (d) metal layer-mixed layer-electrical insulating layer-   (e) mixed layer-electrical insulating layer-mixed layer-   (f) metal layer-mixed layer-electrical insulating layer-mixed    layer-metal layer

A thermal stress relief pad is produced by appropriately using thestructures shown in (a) to (f). For example, a thermal stress relief padis produced such that an electrical insulating material powder is coatedon surfaces of mixed layers of two green compacts and the green compactsare sintered and connected in a state in which the electrical insulatinglayer is disposed therebetween. In this case, the structure shown in (a)or (b) is used as the green compact. For example, a thermal stressrelief pad is produced such that two green compacts, which have halfthickness including the electrical insulating material powder, aresintered and connected in a state in which the electrical insulatinglayer is disposed therebetween. In this case, the structure shown in (c)or (d) is used as the green compact. The structures shown in (a) or (b)can be used as one of the above green compacts. A thermal stress reliefpad can be produced by sintering in a state of green compact having thestructure shown in (e) or (f).

(11) Coating of Electrical Insulating Material Powder on Green Compactof powders

Coating an electrical insulating material powder to the green compact ofonly the mixed powder shown in the above (a) or on the green compact ofthe metal layer and the mixed layer shown in the above (b) can beperformed in a state of a powder or slurry thereof. A method in whichthe electrical insulating material powder is dropped from a sieve to theside of the mixed layer of the green compact which is mounted at top andthen the other green compact is mounted thereon so that the electricalinsulating material powder is disposed therebetween is used.Alternatively, a method in which pasted liquid of the above CMC or theabove PVA is coated on the mixed layer of the green compact and then theother green compact is mounted thereon so that the electrical insulatingmaterial powder is disposed therebetween is used. The slurry of theelectrical insulating material powder is commercial enamel liquid (glazeslurry), organic solvent such as mineral oil, liquid paraffin, alcohol,or acetone, or mixed dispersed liquid of PVA or CMC.

(12) Sintering

The same sintering as that of the third embodiment is used.

Next, the fourth embodiment of the present invention will be describedwith reference to the Figures.

FIGS. 8A to 8C are cross sectional diagrams showing thermal stressrelief pads 401A to 401C for thermoelectric conversion elements. In thethermal stress relief pads 401A to 401C, the metal is copper, and theceramic is alumina and/or enamel frit.

The thermal stress relief pad 401A shown in FIG. 8A has an electricalinsulating layer 402 at a center portion in a thickness direction, andmixed layers 403 having plural mixed layers on both sides of theelectrical insulating layer 402. The mixed layer 403 has a structuresuch that a first mixed layer 431, a second mixed layer 432, and a thirdmixed layer 433 are layered in turn on the side of the electricalinsulating layer 402. The content ratio of copper is small in the firstmixed layer 431, the volume content volume ratio of copper to electricalinsulating material is 1:1 in the second mixed layer 432, and thecontent of copper is large in the third mixed layer 433. The thermalstress relief pad 401B shown in FIG. 8B has a structure such that metallayers 404 made only of copper are layered on both surfaces of thermalstress relief pad 401A shown in FIG. 8A. The thermal stress relief pad401C shown in FIG. 8C has a structure such that a metal layer 404 madeonly of copper is layered on one of the surfaces of the thermal stressrelief pad 401A shown in FIG. 8A. In FIG. 8C, the metal layer 404 madeonly of copper is layered on the bottom surface of thermal stress reliefpad 401A shown in FIG. 8A.

The powders used for producing the above thermal stress relief pads 401Ato 401C are the same as those of the third embodiment.

For example, three method, as shown in FIGS. 9A to 9C, are used as acompacting of powders. In all cases, a zinc stearate powder is coated byelectrostatic coating on an inner wall of the cavity, the above powdersare filled thereto in turn by using a feeder, and then are compacted ata pressure of 700 MPa. These methods can be used for compacting thethermal stress relief pads 401A and 401C shown in FIGS. 8A and 8Cinstead of the thermal stress relief pad 401B shown in FIG. 8B.

FIG. 9A shows a method in which, when compacting is performed onpowders, all used powders are filled and layered so that the powders aresimultaneously and integrally compacted, and then are sintered. FIG. 9Bshows a method in which, when compacting is performed on powders, twogreen compacts having the electrical insulating layer 402, the mixedlayer 403 and the metal layer 404 are obtained and then sintered in astate in which the electrical insulating layers 402 are contacted toeach other. In this case, one of the green compacts may not have theelectrical insulating layer 402. FIG. 9C shows a method in which, aftertwo green compacts having the mixed layer 403 and the metal layer 404are obtained, the electrical insulating layer 402 is formed by coatingthe electrical insulating material powder on the surface of the mixedlayer 403 of one green compact, the other green compact is mounted onthe electrical insulating layer 402 of one green compact, and then theysintered.

The above thermal stress relief pads 401A to 401C can be applied to thethermoelectric conversion module 307 in the same manner as thermalstress relief pads 301A to 301E of the third embodiment.

When the thermal stress relief pads 401A and 401C are used, thethermoelectric conversion elements 305 are connected by a conductivemember corresponding to the metal layer 404, and the surface of themixed layer 403 is contacted to the conductive member.

1. A production method for a sintered metal-ceramic layered compact,comprising steps of: filling and layering a metal powder and a ceramicpowder, or filling and layering a metal powder, a mixed powder of ametal powder and a ceramic powder, and a ceramic powder; forming a greencompact of the layered powders by compacting the layered powders; andsintering a layer including the metal of the green compact at atemperature of lower than a melting point of the metal by heating byirradiation of microwaves in a non-oxidizing atmosphere.
 2. Theproduction method for a sintered metal-ceramic layered compact accordingto claim 1, wherein the production method uses a microwave heatingfurnace provided with a cooling device, and a side of the metal layer ofthe compact is contacted to the cooling device of the microwave heatingfurnace in the step of sintering the green compact.
 3. The productionmethod for a sintered metal-ceramic layered compact according to claim1, wherein the metal is selected from a group consisting of copper,aluminum, silver, and nickel, or a mixture thereof, and the ceramic isalumina or aluminum nitride.
 4. The production method for a sinteredmetal-ceramic layered compact according to claim 1, wherein the ceramicpowder includes at least one low melting point powder selected from agroup consisting of boric acid, anhydrous borax, sodium triboric acid,sodium pentaboric acid, and soda-lime glass, and the low melting pointpowder is mixed in a ratio of not more than 50 mass % in the ceramicpowder.
 5. The production method for a sintered metal-ceramic layeredcompact according to claim 1, wherein the ceramic powder includes atleast one binder selected from a group consisting of methyl cellulose(MC), polyvinyl alcohol (PVA), ammonium alginic acid, carboxymethylcellulose (CMC), hydroxyethyl cellulose (HEC), and polyvinyl pyrrolidone(PVP), and the binder is mixed in a ratio of not more than 1 mass % inthe ceramic powder.
 6. The production method for a sinteredmetal-ceramic layered compact according to claim 5, wherein the mixedpowder of the ceramic powder and the binder is granulated to have aparticle diameter of not more than 150 μm.
 7. The production method fora sintered metal-ceramic layered compact according to claim 1, whereinthe mixed powder of the metal powder and the ceramic powder has two ormore mixed powders which have different compositions from each other,wherein the metal is mixed in a volume not less than that of the ceramicpowder in the mixed powder disposed on the side of the metal layer, andthe ceramic powder is mixed in a volume not less than that of the metalpowder in the mixed powder disposed on the side of the ceramic layer. 8.A production method for a sintered metal-ceramic layered compact,comprising steps of: filling and layering a metal powder and a ceramicpowder, or filling and layering a metal powder, a mixed powder of ametal powder and a ceramic powder, and a ceramic powder; forming a greencompact of the layered powders by compacting the layered powders;presintering a layer including the metal of the compact at a temperaturelower than a melting point of the metal by heating by irradiation ofmicrowaves in a non-oxidizing atmosphere; and resintering thepresintered compact at a temperature lower than a melting point of themetal in a non-oxidizing atmosphere.
 9. The production method for asintered metal-ceramic layered compact according to claim 8, wherein theproduction method uses a microwave heating furnace provided with acooling device, and a side of the metal layer of the compact iscontacted to the cooling device of the microwave heating furnace in thestep of sintering the compact.
 10. The production method for a sinteredmetal-ceramic layered compact according to claim 8, wherein the metal isselected from a group consisting of copper, aluminum, silver, andnickel, or a mixture thereof, and the ceramic is alumina or aluminumnitride.
 11. The production method for a sintered metal-ceramic layeredcompact according to claim 8, wherein the ceramic powder includes atleast one low melting point powder selected from a group consisting ofboric acid, anhydrous borax, sodium triboric acid, sodium pentaboricacid, and soda-lime glass, and the low melting point powder is mixed ina ratio of not more than 50 mass % in the ceramic powder.
 12. Theproduction method for a sintered metal-ceramic layered compact accordingto claim 8, wherein the ceramic powder includes at least one binderselected from a group consisting of methyl cellulose (MC), polyvinylalcohol (PVA), ammonium alginic acid, carboxymethyl cellulose (CMC),hydroxyethyl cellulose (HEC), and polyvinyl pyrrolidone (PVP), and thebinder is mixed in a ratio of not more than 1 mass % in the ceramicpowder.
 13. The production method for a sintered metal-ceramic layeredcompact according to claim 12, wherein the mixed powder of the ceramicpowder and the binder is granulated to have a particle diameter of notmore than 150 μm.
 14. The production method for a sintered metal-ceramiclayered compact according to claim 8, wherein the mixed powder of themetal powder and the ceramic powder has two or more mixed powders whichhave different compositions from each other, wherein the metal is mixedin a volume not less than that of the ceramic powder in the mixed powderdisposed on the side of the metal layer, and the ceramic powder is mixedin a volume not less than that of the metal powder in the mixed powderdisposed on the side of the ceramic layer.
 15. A production method for asintered metal-ceramic layered compact, comprising steps of: filling andlayering a metal powder and a ceramic powder, or filling and layering ametal powder, a mixed powder of a metal powder and a ceramic powder, anda ceramic powder; forming a green compact of the layered powders bycompacting the layered powders; and sintering the compact at atemperature lower than a melting point of the metal in a non-oxidizingatmosphere.
 16. The production method for a sintered metal-ceramiclayered compact according to claim 15, wherein the metal is selectedfrom a group consisting of copper, aluminum, silver, and nickel, or amixture thereof, and the ceramic is alumina or aluminum nitride.
 17. Theproduction method for a sintered metal-ceramic layered compact accordingto claim 15, wherein the ceramic powder includes at least one lowmelting point powder selected from a group consisting of boric acid,anhydrous borax, sodium triboric acid, sodium pentaboric acid, andsoda-lime glass, and the low melting point powder is mixed in a ratio ofnot more than 50 mass % in the ceramic powder.
 18. The production methodfor a sintered metal-ceramic layered compact according to claim 15,wherein the ceramic powder includes at least one binder selected from agroup consisting of methyl cellulose (MC), polyvinyl alcohol (PVA),ammonium alginic acid, carboxymethyl cellulose (CMC), hydroxyethylcellulose (EC), and polyvinyl pyrrolidone (PVP), and the binder is mixedin a ratio of not more than 1 mass % in the ceramic powder.
 19. Theproduction method for a sintered metal-ceramic layered compact accordingto claim 18, wherein the mixed powder of the ceramic powder and thebinder is granulated to have a particle diameter of not more than 150μm.
 20. The production method for a sintered metal-ceramic layeredcompact according to claim 15, wherein the mixed powder of the metalpowder and the ceramic powder has two or more mixed powders which havedifferent compositions from each other, wherein the metal is mixed in avolume not less than that of the ceramic powder in the mixed powderdisposed on the side of the metal layer, and the ceramic powder is mixedin a volume not less than that of the metal powder in the mixed powderdisposed on the side of the ceramic layer.
 21. A production method for athermal stress relief pad for thermoelectric conversion elements,comprising steps of: filling and layering an electrical insulatingpowder (30C) and a mixed powder (30B) of a metal powder and anelectrical insulating powder in turn in a cavity of a die, and forming agreen compact (31) of the layered powders by compacting the layeredpowders, or filling and layering an electrical insulating powder (30C),a mixed powder (30B) of a metal powder and an electrical insulatingpowder, and a metal powder (30A) in turn in a cavity of a die, andforming a green compact (32) of the layered powders by compacting thelayered powders; and contacting an electrical insulating layer, which ismade of the electrical insulating powder (30C) in either the greencompact (31) or the green compact (32), to a surface of an electricalinsulating layer of the electrical insulating powder (30C) in either thegreen compact (31) or the green compact (32); or filling a mixed powder(30B) of a metal powder and a ceramic powder in a cavity of a die, andforming a green compact (33) by compacting the powder, and contacting anelectrical insulating layer, which is made of the electrical insulatingpowder (30C) in either the green compact (31) or the green compact (32),to a surface of the green compact (33), or filling and layering a metalpowder (30A) and a mixed powder (30B) of a metal powder and anelectrical insulating powder in turn in a cavity of a die, and forming agreen compact (34) of the layered powders by compacting the layeredpowders, and contacting an electrical insulating layer, which is made ofthe electrical insulating powder (30C) in either the green compact (31)or the green compact (32), to a surface of the green compact (34); andsintering the green compacts, which are in the above contacting state toeach other, at a temperature lower than a melting point of the includedmetal in a non-oxidizing atmosphere.
 22. The production method for athermal stress relief pad for thermoelectric conversion elements,according to claim 21, wherein the electrical insulating powder is amixed powder (30C1), a mixed powder (30C2), or a glass frit powder(30C3), wherein the mixed powder (30C1) is composed of one of an aluminapowder and an aluminum nitride powder, and one low melting pointelectrical insulating powder selected from a group consisting of boricacid, sodium boric acid, and soda-lime glass, the low melting pointelectrical insulating powder being mixed in a ratio of not more than 50mass %, the mixed powder (30C2) is composed of one of an alumina powderand an aluminum nitride powder and a glass frit which is mixed in aratio of not less than 0.1 mass %, and the metal powder (30A) isselected from a group consisting copper, aluminum, silver, and nickel,or a mixture thereof.
 23. A production method for a thermal stressrelief pad for thermoelectric conversion elements, comprising steps of:filling and layering a mixed powder (30B) of a metal powder and anelectrical insulating powder, an electrical insulating powder (30C), anda mixed powder (30B) of a metal powder and an electrical insulatingpowder in turn in a cavity of a die, or filling and layering a metalpowder (30A), a mixed powder (30B) of a metal powder and an electricalinsulating powder, an electrical insulating powder (30C), a mixed powder(30B) of a metal powder and an electrical insulating powder, and a metalpowder (30A) in turn in a cavity of a die; forming a green compact ofthe layered powders by compacting the layered powders; sintering thegreen compact at a temperature lower than a melting point of theincluded metal powder in a non-oxidizing atmosphere; and removing a sidesurface portion of the sintered compact by cutting or by polishing. 24.The production method for a thermal stress relief pad for thermoelectricconversion elements, according to claim 23, wherein the electricalinsulating powder is a mixed powder (30C1), a mixed powder (30C2), or aglass frit powder (30C3), wherein the mixed powder (30C1) is composed ofone of an alumina powder and an aluminum nitride powder, and one lowmelting point electrical insulating powder selected from a groupconsisting of boric acid, sodium boric acid, and soda-lime glass, onelow melting point electrical insulating powder being mixed in a ratio ofnot more than 50 mass %, the mixed powder (30C2) is composed of one ofan alumina powder and an aluminum nitride powder and a glass frit whichis mixed in a ratio of not less than 0.1 mass %, and the metal powder(30A) is selected from a group consisting copper, aluminum, silver, andnickel, or a mixture thereof.
 25. A production method for a thermalstress relief pad for thermoelectric conversion elements, comprisingsteps of: filling and layering an electrical insulating material powder(40A) for an electrical insulating layer and a mixed powder (40B) of ametal powder and an electrical insulating material powder in a die, orfilling and layering an electrical insulating material powder (40A) foran electrical insulating layer, a mixed powder (40B) of a metal powderand an electrical insulating material powder, and a metal powder (40C)in a die; forming a green compact of the layered powders by compactingthe layered powders; and sintering the green compact at a temperaturelower than a melting point of the included metal powder in anon-oxidizing atmosphere, wherein the metal powder is selected from agroup consisting of copper, aluminum, silver and nickel, or a mixturethereof, the electrical insulating material powder (40A) is selectedfrom a group consisting of a glass frit (40A1) and a mixed powder (40A2)of a ceramic powder and a glass frit, the ceramic powder being composedof alumina or aluminum nitride, the electrical insulating materialpowder (40A) included in the mixed powder (40B) is selected from a groupconsisting of a ceramic powder, the glass frit (40A1), and a mixedpowder (40A2) of a ceramic powder and a glass frit, the ceramic powderbeing composed of alumina or aluminum nitride.
 26. The production methodfor a thermal stress relief pad for thermoelectric conversion elementsaccording to claim 25, wherein the electrical insulating material powder(40A) is a mixed powder (40A2) of the ceramic powder and the glass frit,and the glass frit is mixed in a ratio of not less than 0.1 mass % inthe mixed powder (40A2).
 27. The production method for a thermal stressrelief pad for thermoelectric conversion elements according to claim 25,wherein the mixed powder (40B), the electrical insulating materialpowder (40A) and the mixed powder (40B) are layered in turn in the diein the step of filling and layering powders, or the metal powder (40C),the mixed powder (40B), the electrical insulating material powder (40A),the mixed powder (40B), and the metal powder (40C) are layered in turnin the die in the step of filling and layering the powders, and thelayered compact of the powders are integrally compacted in the step ofcompacting.
 28. The production method for a thermal stress relief padfor thermoelectric conversion elements according to claim 25, whereinthe mixed powder (40B) and the electrical insulating material powder(40A) are layered in turn in the die in the step of filling and layeringpowders, or the metal powder (40C), the mixed powder (40B) and theelectrical insulating material powder (40A) are layered in turn in thedie in the step of filling and layering powders, the layered compact ofthe powders are integrally compacted in the step of compacting, wherebytwo green compacts are obtained, and the green compacts are sintered ina state in which surfaces of layers of the electrical insulatingmaterial powder (40A) are contacted to each other in the sintering step,thereby being connected.
 29. The production method for a thermal stressrelief pad for thermoelectric conversion elements according to claim 25,wherein the electrical insulating material powder (40A) includes atleast one binder selected from a group consisting of methyl cellulose(MC), polyvinyl alcohol (PVA), ammonium alginic acid, carboxymethylcellulose (CMC), hydroxyethyl cellulose (HEC), and polyvinyl pyrrolidone(PVP), wherein the binding agent is mixed in a ratio of not more than 1mass %.
 30. The production method for a thermal stress relief pad forthermoelectric conversion elements according to claim 25, wherein thebinder is mixed into the electrical insulating material powder (40A) ina middle portion layer in a thickness direction, and the mixed powder isgranulated so as to have a particle diameter of not more than 150 μm.31. The production method for a thermal stress relief pad forthermoelectric conversion elements according to claim 25, wherein themixed powder (40B) has two or more mixed powders which have differentcomposition from each other, wherein the metal powder (40C) is mixed ina volume not less than that of the electrical insulating material powder(40A) on the side of the metal layer formed on an end face, and theelectrical insulating material powder (40A) is mixed in a volume morethan that of the metal powder (40C) in a electrical insulating layerformed at a middle portion in a thickness direction.
 32. A productionmethod for a thermal stress relief pad for thermoelectric conversionelements, comprising steps of: filling a mixed powder (40B) of a metalpowder and an electrical insulating material powder in a die, or fillingand layering a mixed powder (40B) of a metal powder and an electricalinsulating material powder, and a metal powder (40C) in turn in a die;forming a green compact of the layered powders by compacting the layeredpowders, whereby two green compacts of the layered powders are obtained;coating an electrical insulating material powder (40A) on a surface of alayer of the mixed powder (40B) of one of the green compacts; andconnecting the green compacts via the electrical insulating materialpowder (40A) by sintering.
 33. The production method for a thermalstress relief pad for thermoelectric conversion elements, according toclaim 32, wherein the electrical insulating material powder (40A) coatedon a surface of a layer of the mixed powder (40B) is dispersed in aliquid so as to be made into slurry.
 34. The production method for athermal stress relief pad for thermoelectric conversion elementsaccording to claim 32, wherein the mixed powder (40B) has two or moremixed powders which have different composition from each other, whereinthe metal powder (40C) is mixed in a volume not less than that of theelectrical insulating material powder (40A) on the side of the metallayer formed on an end face, and the electrical insulating materialpowder (40A) is mixed in a volume more than that of the metal powder(40C) in an electrical insulating layer formed at a middle portion in athickness direction.